Preheat the oven to 375° F. Spray a 3-quart baking dish with nonstick spray. Set aside.
Bring a large pot of water with 1 Tablespoon of Kosher salt to a boil. Add the macaroni and cook according to the directions on the package, for 9 to 10 minutes. Drain and rinse well with cold water; drain completely. Set aside.
Melt 6 tablespoons of butter in a large (4-quart) pot; whisk in the flour. Cook over medium heat for 2 minutes, stirring with a whisk. Whisk in the milk and cook for several more minutes, until bubbly and thickened.
Remove pan from heat; add 3 cups of the Cheddar cheese, all of the Gruyère cheese, ½ tablespoon salt, pepper, and nutmeg. Stir until all of the cheese is melted and the sauce is creamy and smooth.
Add the cooked macaroni and stir well. Pour into a 3-quart baking dish.
Top macaroni and cheese with remaining 1-½ cups Cheddar Cheese and Panko breadcrumbs mixed. Dot with remaining butter.
May be prepared in advance up to this point, covered, and refrigerated. (See Tip 4)
Bake for 40 to 45 minutes, or until the sauce is bubbly and the top is golden brown. (See Tip 5)
Yield: 10 to 12 Servings.